A transparent plastic substrate has an excellent low birefringence which is the basic performance in optical properties, and a great attention is recently paid thereto.
The transparent plastic substrate is generally produced by a cast molding method comprising pouring a polymerizable liquid material such as polyallyl carbonate, polyolpoly(meth)acrylate or epoxy acrylate into a cavity of a mold and radical polymerizing the same in the mold to obtain a transparent plastic substrate as disclosed in, for example, Japanese Patent Application (OPI) Nos. 130450/83 and 137150/83 (the term "OPI" as used herein means a "published unexamined Japanese patent application").
In addition, a method for conducting photopolymerization in a mold constituted by glass plates at both sides (Japanese Patent Application No. 202557/85), a method of thermal polymerization after casting a liquid resin into a mold under vacuum, a method of thermal polymerization under pressure for a liquid resin casted (Japanese Patent Application (OPI) No. 203414/85), and so on are known.
Conventional cast molding method of the transparent plastic substrate requires about 24 hours from the pouring of a reactive liquid material into the mold used to completion of molding of the transparent plastic substrate. Therefore, the efficiency of use of the mold is extremely low, and the number of stampers to transfer signals, grooves for trucking and so on is increased. This is a serious problem to the productivity of the transparent plastic substrate.
Furthermore, as a method for increasing the efficiency of use of the mold, it is sufficient to shorten the molding time. However, for example, if radical polymerization is completed in the mold by light energy, polyfunctional (meth)acrylate compounds produce a residual strain by the shrinkage during polymerization or curing reaction.
In demolding the transparent plastic substrate from the mold constituted by a glass, a metal, a rubber or the like after the completion of molding, the transparent plastic substrate is broken or causes cracking, or the mold is broken. Those are the practical problems. Accordingly, it has been desired to improve the defects.